He is still marking territory, or has a problem the vet hasn't found. Has the vet ruled out bladder or kidney stones? Is the litter box clean - meaning scooped twice a day - and do you have more than one litter box? Add the second or third box. If they are covered, take off the lids. Is there a possibility that there are stray cats outside, who are spraying and he is reacting to that? Have you thoroughly cleaned the areas he's sprayed with an enzymatic cleaner like Nature's Miracle? If he can smell the spots he's going to continue to mark over them. Do they both have "territory" to call their own? A cat tree with 4 levels can provide that. Put it in front of a sunny window. Has he been declawed? A declawed cat can develop pain in their paws, like bone chips, and that can cause spraying too. If you've done all of this, as the vet about Prozac for him. It can do wonders for cats that are anxious - and many anxious cats begin to spray to mark territory. It's cheap and generally very effective as that last resort.